LCD panels are finding increasing applicability in sophisticated display devices. A drawback to their further exploitation, however, is their relatively limited resolution.
The resolution of LCD panels is limited both by interconnection constraints and by the electrical properties of the liquid crystal material itself. Taking this latter limitation first, in any multiplexed LCD display, each cell must be electrically refreshed periodically, typically 30 or 60 times a second, to maintain its desired state. This is effected by repetitively scanning down the panel, refreshing each row in turn. The greater the resolution of a panel, the greater the number of rows that must be refreshed at this rate. Beyond a certain limit, the period allotted to refreshing each row becomes too short to refresh it effectively. Thus, a minimum refresh period limits the number of rows that can be refreshed at the requisite rate. This number is about 250-300 rows with current liquid crystal materials.
In the prior art, displays with twice this number of rows have been achieved by duplicating the refresh circuitry so that half the rows of the panel are refreshed by one circuit and half are refreshed by the other. Thus, at any instant, two rows are being refreshed--one by one circuit and one by the other. However, this technique still only permits 500 or so rows of resolution. Truly high resolution applications demand substantially more rows.
The obstacle to refreshing more than 500 rows is the interconnection limitation. The refresh circuitry must connect to each column of pixels on the display. There may be 640 or more such columns. By partitioning the display into top and bottom portions, the two requisite 640 wire connections can be made--one along the top of the display and one along the bottom. However, this partitioning approach cannot be extended to a three- or more way division because there is no way to make the requisite interconnect to intermediate portions of the display.
The interconnect limitation is generally accepted to be an absolute bar to arbitrarily-high resolution LCD displays, as noted in "Scanning Limitations of Liquid Crystal Displays" by P. M. Alt et al, IEEE Trans. Electron Devices, Vol. ED-21, pp. 146-155 (1974); and "Ultimate Limits for Matrix Addressing of RMS-Responding L. C. D.'s" by J. Nehring et al, IEEE Trans. Electron. Devices, Vol. ED-26, p. 795-802 (1979).
It is a principal object of the present invention to provide a display that circumvents both the electrical and interconnect limitations, permitting fabrication of LCD displays of virtually unlimited resolution.
In accordance with the present invention, a composite display is fabricated with a plurality of panels. Each panel has active rows and inactive rows. The panels are stacked and aligned so that each active row is in alignment with inactive rows in all the other panels of the stack. The parallax problem inherent with this stacked cell approach is eliminated by a lens system that collimates light illuminating the stacked assembly. The limited viewing angle associated with collimated illumination is overcome by an exit optic that may either focus or disperse the exiting light for viewing.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying drawings.